Here we consider the constraints that arise from unitarity. Since the
gravitational EFTs of interest are weakly coupled, we limit ourselves
to perturbative unitarity as discussed for example in
Refs. [1]}, [2]}. It would be of
course be very interesting to implement unitarity nonperturbatively
along the lines of Refs. [3]}, [4]}. In four
dimensions this would also require understanding the constraints of
unitarity at the level of IR-finite observables; see
e.g. Refs. [5]}, [6]} for a recent discussion. Luckily, for
our purpose of investigating the leading-order corrections to the
gravitational EFT these subtle but important issues are irrelevant.
Here we primarily follow the discussion of
Ref. [2]} but modify it to account for
differing helicity configurations.
